Trigger SendAccount_Trigger on Account (after insert) {
    for(Account a : Trigger.new){
        SendAccount.createAccount(a.name, a.Phone, a.Website);
    }
}

public class SendAccount {

    String clientId = '***************************************************************';
    String clientsecret ='*******************';
    String username='fake@fake.com';
    String password = '******************************************';
    String accesstoken_url = 'https://login.salesforce.com/services/oauth2/token';
    String authurl = 'https://login.salesforce.com/services/oauth2/authorize';

    public class deserializeResponse {

        public String id;
        public String access_token;

    }

    public String ReturnAccessToken(SendAccount Acc){

        String reqbody = 'grant_type=password&client_id='+clientId+'&client_secret='
        +clientSecret+'&username='+username+'&password='+password;

        Http h= new Http();
        HttpRequest req= new HttpRequest();
        req.setBody(reqbody);
        req.setMethod('POST');
        req.setEndpoint('https://na50.salesforce.com/services/oauth2/token');

        HttpResponse res=h.send(req);

        deserializeResponse resp1= (deserializeResponse)JSON.deserialize(res.getBody(),deserializeResponse.class);

        return resp1.access_token;
    }

    @future(callout=true)
    public static void createAccount(String Accname, String Phone, String Website){

        SendAccount acc1 = new SendAccount();

        String accessToken = acc1.ReturnAccessToken(acc1);

        if(accessToken!=null){

            String endPoint='https://na50.salesforce.com/services/data/v42.0/sobjects/Account';

            String jsonstr='{"Name":"'+ Accname +'","Phone":"'+ Phone +'","Website":"'+ Website +'"}';

            Http h2 = new Http();
            HttpRequest req2 = new HttpRequest();
            req2.setHeader('Authorization','Bearer ' + accessToken);
            req2.setHeader('Content-Type','application/json');
            req2.setHeader('accept','application/json');
            req2.setBody(jsonstr);
            req2.setMethod('POST');
            req2.setEndpoint(endPoint);

            HttpResponse res2 = h2.send(req2);

            deserializeResponse deresp=(deserializeResponse)System.JSON.deserialize(res2.getBody(),deserializeResponse.class);
        }
    }
}

trigger SendOpp_Trigger on Opportunity (before insert) {
    for(Opportunity o : Trigger.new){
        SendOpportunity.createOpportunity(o.name, o.Account_External_Id__c, o.CloseDate, o.StageName);
    }
}

public class SendOpportunity {

    String clientId = '***************************************************************';
    String clientsecret ='*******************';
    String username='fake@fake.com';
    String password = '******************************************';
    String accesstoken_url = 'https://login.salesforce.com/services/oauth2/token';
    String authurl = 'https://login.salesforce.com/services/oauth2/authorize';

    public class deserializeResponse {

        public String id;
        public String access_token;

    }

    public String ReturnAccessToken(SendOpportunity Opp){

        String reqbody = 'grant_type=password&client_id='+clientId+'&client_secret='
        +clientSecret+'&username='+username+'&password='+password;

        Http h= new Http();
        HttpRequest req= new HttpRequest();
        req.setBody(reqbody);
        req.setMethod('POST');
        req.setEndpoint('https://na50.salesforce.com/services/oauth2/token');

        HttpResponse res=h.send(req);

        deserializeResponse resp1= (deserializeResponse)JSON.deserialize(res.getBody(),deserializeResponse.class);

        return resp1.access_token;
    }

    @future(callout=true)
    public static void createOpportunity(String oppName, String acctId, Date d, String stage){

        SendOpportunity opp1 = new SendOpportunity();

        String accessToken = opp1.ReturnAccessToken(opp1);

        if(accessToken!=null){

            String endPoint='https://na50.salesforce.com/services/data/v42.0/sobjects/Opportunity';

            String jsonstr='{"Name":"'+ oppName +'","Account_External_Id__c":"'+ acctId +'","CloseDate":"'+ d +'","StageName":"'+ stage +'"}';

            Http h2 = new Http();
            HttpRequest req2 = new HttpRequest();
            req2.setHeader('Authorization','Bearer ' + accessToken);
            req2.setHeader('Content-Type','application/json');
            req2.setHeader('accept','application/json');
            req2.setBody(jsonstr);
            req2.setMethod('POST');
            req2.setEndpoint(endPoint);

            HttpResponse res2 = h2.send(req2);

            deserializeResponse deresp=(deserializeResponse)System.JSON.deserialize(res2.getBody(),deserializeResponse.class);
        }
    }
}

Malformed JSON: Expected '{' at the beginning of object

deserializeResponse deresp = (deserializeResponse)System.JSON.deserialize(res2.getBody(),deserializeResponse.class);

'{"Name":"'+ Accname +'","Phone":"'+ Phone +'","Website":"'+ Website +'"}';

'{"Name":"'+ oppName +'","Account_External_Id__c":"'+ acctId +'","CloseDate":"'+ d +'","StageName":"'+ stage +'"}';